Gas discharge lamps such as fluorescent lamps are presently commonly used in homes and commercial buildings. The lamps commonly contain a phosphor coated glass tube confining an ionizable gas and a small amount of mercury and include electron-emitting cathodes and electrical terminals at each end. Upon application of proper electrical voltages to the terminals, the gas becomes ionized and an electrical arc is established between the cathodes thereby energizing the phosphor coating which thereby fluoresces generating diffused light. It is known that gas discharge lamps are most efficiently operated when driven with an alternating current of a high frequency typically over 20 Khz.
However, most electrical power sources provide electric current at low frequencies. Alternating current is at 60 Hz in North America and 50 Hz in most other continents. These frequencies have been used in the past and are adequate for driving gas discharge lamps by, for example, stepping up the voltage to an appropriate level for causing the necessary arching for starting the lamp, then limiting the current to the lamp for proper drive levels. To achieve the higher efficiencies that are available by driving the lamps at high frequencies, typically, ballast circuits include a means for creating direct current and thereafter some oscillating or high frequency driver circuit is provided for creating a high frequency signal over 20 KHz. Prior oscillating circuits include series fed parallel resonant or push-pull circuits incorporating two transistors and examples thereof are disclosed in U.S. Pat. Nos. 5,177,408 and 4,277,726. Other such circuits accomplish the high frequency oscillation via integrated components and examples of such oscillating circuits are shown in U.S. Pat. Nos. 5,124,619, 5,178,234, 4,985,664 and 4,717,863. These circuits that first rectify the alternating power source signal to direct current and, thereafter, produce the high frequency lamp driving signal are commonly referred to in the industry as unmodulated ballast driver circuits and are most common in the industry at the present time.
However, there are significant drawbacks and shortcomings associated with the unmodulated lamp driving circuits. For example, component and manufacturing costs associated with the direct current power source and integrated components are generally relatively high thereby sometimes making the unmodulated driver circuit overly costly even though the efficiencies thereof are higher. Typically, in addition to the larger capacitance needed for creating the direct current, an inductance is also then needed for correcting the power factor. Further, numerous supporting components are needed for the direct current producing circuit and the integrated components used for creating the high frequency driving signal.
Modulated lamp driving circuits are also known and, for example, are disclosed in U.S. Pat. No. 3,579,026. Although these circuits solve some of the problems associated with unmodulated lamp driving circuits, they too have shortcomings and drawbacks. The modulated circuits of the past, for example, rectify the alternating power source signal to a substantially unfiltered rectified direct current signal at 120 Hz and, thereafter, modulate a high frequency signal on the 120 Hz signal. Unfortunately, during the zero voltage valleys of the rectified signal, the resonant circuit stops switching and the electric arc across the gas discharge lamp is temporarily discontinued. Unfortunately, this discontinuous mode of operation places added stress on the electrical components since these components are constantly being subjected to "start-up" conditions.
In addition to the discontinuous mode of operation, prior modulated gas discharge lamp driving circuits tend to be inefficient because of inefficient switching of the transistors in the resonant circuit. Typically, these transistors exhibit switching losses due to slow switching times and co-conduction. Although the circuit can normally be designed to prevent such losses during full load conditions, they are not readily capable of operating efficiently during both load and no load conditions and, thus, still exhibit co-conduction and switching losses. Further yet, modulated discharge lamp driving circuits of the past fall short of properly shielding the power source lines from conducted electromagnetic interference (EMI) and also are incapable of dimming the discharge lamps continuously over a large range of light output.
Accordingly, a need exists for a gas discharge lamp driving circuit that solves problems associated with prior such circuits and which further exhibits a high efficiency in terms of lumens output per watt input, has a high power factor, low harmonic distortion, provides sufficient EMI shielding, and which further is relatively inexpensive to manufacture in terms of component costs and manufacturing assembly time.